Due to the explosive growth of mobile network traffic and signaling, mobile operators are in need of carrier-grade wireless local area network (WLAN) deployment to instantly offload mobile data traffic, as proposed by a well-known market research company, Infonetics Research. The mobile operators analyzed the mobile data traffic of the user equipment, and found that the main flows occur in hot spots and indoors at a certain time. The data type in the future will also be video and audio streams. Many companies such as Alcatel-Lucent, Ericsson, Nokia Siemens, etc., put carrier-grade wireless local area networks into small-cell product blueprints. The wireless local area network vendors also actively develop related technologies, such as Hotspot 2.0, hoping for a place in the offloading market.
Currently, when the UE is used practically, the UE moves with the user. Therefore, the UE may switch between different networks to maintain communication quality, such as, a handover from the cellular network to a wireless local area network. However, in this event, all the bearers (radio bearers, S1 bearers and S5 bearers) of the UE in the 3GPP access network are deleted and the mobility state of the UE is transferred to DEREGISTERED. Deleted means that the resources reserved for the EPS bearers are released (where applicable) and the context information of the bearers are deleted in the UE, base station and MME.
However, when the UE is handed over from the wireless local area network to the cellular network, the UE needs to restart a handover attachment procedure. It may take a lot of time to execute the handover attachment procedure for handling handover messages. Therefore, solving the network congestion and simplifying the handover procedure in multi-radio access technology (multi-RAT) are still urgent problems.